Optical systems for UV photometers often times provide very low signals. This is particularly true when interference filters are employed as the wavelength determining devices. Ultra violet interference filters typically display peak transmissions of about 10%. This results in a signal loss of about 90% when ultra violet radiation passes through an interference filter. In order to operate at these low signal levels, ultra violet optical systems employing interference filters must generally incorporate very sensitive detectors, normally expensive photomultipliers which also require high voltage power supply. In addition, the sensitivity of ultra violet photometers is adversely affected by any instability (noise) arising from the emission source. Ultra violet emission sources are generally gas discharge lamps, all of which suffer to a greater or lesser degree from arc instability. Instrument drift is also encountered as a result of the accumulation of contaminates on the sample cell walls and windows. Also, spectral overlap is considerably more severe in the ultra violet than in the infrared and thus ultra violet photometers are generally characterized by low discrimination ratios and thus may have difficulty in distinguishing between various constituents in a sample gas. Finally, if a sector disc is employed for chopping of the beam, attendant problems of motor stability, mechanical noise and bearing longevity are encountered.